The disclosed invention is directed generally to transmit/receive circuits for active array antennas, and is directed more particularly to a closed loop transmit/receive circuit.
An active antenna array is a directive antenna system comprising, for example, individual antenna elements whose antenna beam direction (i.e., pointing direction) is controlled by the relative phases of the signals respectively radiated or received by the antenna elements. In particular, for transmission, the relative phases of the energy radiated by the individual antenna elements is controlled to define the transmit beam direction of the antenna. For receiving, the relative phases of the energy received by the individual elements is controlled to define the receive beam direction of the antenna. Associated with each antenna element is a transmit/receive circuit that typically includes (a) a transmit channel having a gated transmit RF power amplifier and an associated phase shifter, and (b) a receive channel having a low noise receive amplifier and an associated phase shifter.
Pursuant to efforts to integrate the circuitry for active arrays, transmit/receive circuits have been implemented in monolithic microwave integrated circuits (MMICs) wherein each transmit/receive circuit is an integrated circuit that includes a phase shifter, a power amplifier, a low noise amplifier, and transmit/receive switches. The main shortcomings of such transmit/receive MMICs include problems in yield, cost, reproducibility, long-term stability, manufacturability, performance, power efficiency, gain, and device matching. Some of the shortcomings are compensated by pre-distorting the phase commands provided to the phase shifters of the transmit/receive MMICs, whereby the relation between phase command and desired phase shift is not linear. This technique, however, requires rigorous characterization of the transmit/receive circuits during manufacture, and the storage of large amounts of calibration data. Moreover, there was the possibility of variations of the MMICs with time and/or temperature that would make the calibration data inaccurate.